This tutorial guide covers only basic linux server security tips intended for linux learners. I am writing this guide assuming that you are running Centos 5 or later versions.
Install Firewall (APF or CSF Firewall with BFD) ModSecurity (Web application firewall) ModEvasive (Prevent DDOS attacks) Harden SSH server Fix Open DNS Recursion Install RKhunter Install ClamAV (Antivirus) XInet Servers Hardening (Disable Telnet/Finger or unwanted services) Securing PHP PortsEntry (tool to detect portscans) Harden host.conf (against IP spoofing) Check User Uploaded files Secure /tmp Folders (noexec, nosuid)
Install Firewall
The very first first step on securing a server is installing a firewall (atleast IP tables based) to close all unused or unwanted ports. Once the firewall is installed it is often considered 50% of work done. You can install CSF firewall or APF firewall. Often BFD (brute force detection) utilities comes with firewall.
We will install CSF (Config security firewall) as it is easy to install with plenty of features and easily integrated to CPanel (if you are running)
wget http://www.configserver.com/free/csf.tgz tar zxf csf.tar.gz sh /csf/install.sh
Follow the installer and once installed, you can start the firewall.
csf -s // start the firewall csf -r // restart the firewall csf -f // flush the rules or stop the firewall.
You can see the full installing tutorial here
Harden SSH server
Very often you will see SSH attacks from various bots trying to get access to your server by connected to port 22 with unlimited number of login attempts to break in to your system. Imagine attacks coming from different IPs can put lot of load in you server. You can trace those failed attempts by checking your log file
cat /var/log/secure cat /var/log/messages
To harden your SSH server,
- Run SSH on other port rather than default port 22
- Disable Root login
- Use only protocol 2
- Enable Public key authentication.
Disable Telnet & Other Unused Services
You may want to disable services like telnet, finger and other unwanted services running on your server with xinet.
nano /etc/xinetd.d/telnet // OR nano /etc/xinetd.d/krb5-telnet
look for lines disable=no and change to disable=yes
chkconfig telnet off
Hardening PHP for Security
PHP is the most popular scripting language for apache and mysql. You will need to disable system level functions in the php configuration file.
nano /usr/local/lib/php.ini
Look for the lines and make sure you have the lines as below..
disable_functions = exec,system,shell_exec,passthru register_globals = Off expose_php = Off magic_quotes_gpc = On
It is best to keep magic_quotes to on as otherwise you forms using POST may be used for SQL injection attacks.
Disable Open DNS Recursion (DNS Server)
If you are running bind DNS server, then you might want to check your dns server statistics with dnstools.com. You dont want to allow recursive lookups to performed on your server other than local IP. It can also slowdown your server.
nano /etc/named.conf
Under Options { place a line
Options {
recursion no;
.....
Then restart the bind
service named restart
You will also need to restrict zone transfers and notifications if you are running Bind 9. Refer to: dns server hardening
Install Mod_Security
ModSecurity is a free open source web application firewall which can help you to guard against LFI (local file inclusion attacks) and SQL injection vulnerabilities.
CPanel Installation:
Just go to Cpanel WHM > Plugins > Enable Mod_Security > Save
Source Installation:
That should install mod security in your cpanel. Under apache it should show under installed modules if you run test.php with phpinfo() in it. Try adding some mod security rules. Installing mod_security could be sometimes complicated. Dont use apxs for compiling mod_security as it causes number of problems.
Note: Mod_security needs libxml2 and http-devel libraries before it can be installed. It also requires mod_unique_id enabled in apache modules. To install mod_unique_id, you have to place
LoadModule unique_id_module modules/mod_unique_id.so
in your httpd.conf file.
yum install libxml2 libxml2-devel httpd-devel
Download the latest version of mod_security for apache2 from http://www.modsecurity.org
wget http://www.modsecurity.org/download/modsecurity-apache_2.1.7.tar.gz tar zxf modsecurity-apache_2.5.4.tar.gz cd modsecurity-apache_2.5.4 cd apache2
Then
If you cannot find ./configure then you will need to edit Makefile and make change to top_dir = /usr/lib/httpd (for centos)
make make install
Next, copy the rule files depending on which you want (you can also select minimal rules file which comes with source). Make a directory named modsecurity under /etc/httpd/conf and copy all the modsecurity rules there. Finally include those files in the httpd.conf file
# /etc/httpd/conf/httpd.conf
LoadModule unique_id_module modules/mod_unique_id.so LoadFile /usr/lib/libxml2.so LoadModule security2_module modules/mod_security2.so Include conf/modsecurity/*.conf
Then
/etc/init.d/httpd restart
Log Files
Watch for log files to detect any errors or intrusion activity
/var/log/httpd/modsec_audit
/var/log/httpd/error_log
If you get any errors, i have compiled a list of errors while compiling. see here
Install Mod_Evasive
ModEvasive module for apache offers protection against DDOS (denial of service attacks) in your server.
wget http://www.zdziarski.com/projects/mod_evasive/mod_evasive_1.10.1.tar.gz tar zxf mode_evasive-1.10.1.tar.gz cd mod_evasive
then run the following command for apache2…
> /usr/sbin/apxs -cia mod_evasive20.c
Once mod evasive is installed, place the following lines in your /etc/httpd/conf/httpd.conf
<IfModule mod_evasive20.c> DOSHashTableSize 3097 DOSPageCount 2 DOSSiteCount 50 DOSPageInterval 1 DOSSiteInterval 1 DOSBlockingPeriod 10 </IfModule>
Follow the instructions in the README for more tuning of mod_evasive. This will compile, install and activate the module in your server.
Install RkHunter (Rootkit)
RkHunter is a rootkit scanner scans for vulnerabilities, insecure files, backdoors in your system and reports it so that you can further harden the server. Installing RkHunter is very easy!
yum install rkhunter
To run checks in your system
rkhunter –checkall
OR
rkhunter -c
You can find what command options are available under rkhunter by issuing this help command
> rkhunter –help
Install PortsEntry
Portsentry is a tool to detect port scans and log it. Download the sorce package of portsentry from sourceforge.net
wget http://path/to/portsentry-1.2.tar.gz tar zxf portsentry-1.2.tar.gz make linux make install
If you get errors like while compiling
make linux SYSTYPE=linux Making gcc -O -Wall -DLINUX -DSUPPORT_STEALTH -o ./portsentry ./portsentry.c \ ./portsentry_io.c ./portsentry_util.c ./portsentry.c: In function 'PortSentryModeTCP': ./portsentry.c:1187: warning: pointer targets in passing argument 3 of 'accept' differ in signedness ./portsentry.c: In function 'PortSentryModeUDP': ./portsentry.c:1384: warning: pointer targets in passing argument 6 of 'recvfrom' diffe r in signedness ./portsentry.c: In function 'Usage': ./portsentry.c:1584: error: missing terminating " character ./portsentry.c:1585: error: 'sourceforget' undeclared (first use in this function) ./portsentry.c:1585: error: (Each undeclared identifier is reported only once ./portsentry.c:1585: error: for each function it appears in.) ./portsentry.c:1585: error: expected ')' before 'dot' ./portsentry.c:1585: error: stray '\' in program ./portsentry.c:1585: error: missing terminating " character ./portsentry.c:1595: error: expected ';' before '}' token make: *** [linux] Error 1
To fix:
Open portsentry.c and look for the following line. There will be a extra carriage return breaking the line and you have to delete the carriage return and make single line. It should look like below.
printf (“Copyright 1997-2003 Craig H. Rowland <craigrowland at users dot sourceforget dot net>\n”);
Then run make and make install. That should fix it!
To launch portsentry
/usr/local/psionic/portsentry/portsentry -stcp /usr/local/psionic/portsentry/portsentry -sudp
check the log files /var/log/secure on what portsentry is active or not.
Prevent IP Spoofing
IP spoofing is a security exploit and can be prevented from placing nospoof on in host.conf file. Edit the host.conf file and place the following lines. If you run dns bind, give it preference.
order bind,hosts nospoof on
Install ClamAV
Antivirus protection is the last thing you need for your security to protect against worms and trojans invading your mailbox and files! Just install clamav (a free open source antivirus software for linux). More information can be found on clamav website
yum install clamav
Once you have installed clamav in your centos…here are some of the basic commands using the software..
1. To update the antivirus database
> freshclam
2. To run antivirus
clamav -r /home
3. Running as Cron Daily Job
To run antivirus as a cron job (automatically scan daily) just run crontab -e from your command line. Then add the following line and save the file.
02 1 * * * root clamscan -R /var/www
This will run the cron job daily @ 1.02 AM by scanning the public html. You can change the folder to whatever you want for mail etc.
Thats it! Always keep an eye for log files for any attacks or error messages!
